This invention relates to the positioning and tracking of the read/write head in an information storage system utilizing optically readable media. More specifically, the optical head utilizes a seek system of open loop coarse positioning over a band of tracks on the media disk together with a closed loop servo system to obtain accurate and precise positioning of the head over the desired track within the band. Once the desired track is found, the invention combines a unique media data format with a closed loop error detection circuit to cause the optical head to follow the track during a revolution of the media disk despite track runout and other inherent variations.
It has been known for some time that digital information systems with high storage densities can be achieved through the use of optical storage media. Implementation of this technology has been hindered, however, by a number of technical barriers. In particular, difficulty has been encountered in quickly achieving precise head-to-track alignment when moving along the radial axis of the media disk to find the addressed track, in staying in alignment with the rotating track, and in maintaining synchronization between the data on the various track sectors with the data recovery and decoding circuitry. These difficulties are more pronounced in optical media storage systems because a smaller amount of media area is allocated to each information track and thus more precise head to track alignment is required. A small media disk may contain over 12,500 tracks in a radial space of approximately twenty-three millimeters and have a track to track spacing of less than 2 micrometers. For a high performance system it is necessary to access the addressed track in a short time and then to accurately follow it so that substantive data may be written or read.
Accordingly, it is the primary object of the present invention to provide positioning and tracking apparatus for the read/write head of an optically responsive information storage system which will overcome the deficiencies and high cost considerations of past systems and which will rapidly seek, find and follow the addressed track.
A second object of the invention is to create a media storage disk having circular or helical data carrying tracks which are grouped into a number of coarse bands, any one band of which can be accessed by an open loop coarse head positioning apparatus.
A third object of the invention is to provide fine head positioning and tracking apparatus which will seek and follow a single addressed track within the band.
A further object of the invention is to eliminate the cumbersome and duplicative "pre-groove" system of tracking found in the prior art and utilize for such purpose a plurality of fields of off-track center line position (servo) data bits embedded in the media and interspersed at intervals between other data fields.
A still further object of the invention is to provide a track format for the media which will implement the fine positioning and tracking objectives and which at the same time will insure the precise synchronization of the data decoding circuitry with the data recorded on the media disk.
Another object of the invention is to provide a tracking servo system which is invariant (to the first order) to discrepancies in the far field pattern of laser light sources enabling maximum flexibility in laser diode selection and reduced cost and complexity of the read/write optical system.
Still another object of the present invention is to provide a head tracking system that substantially reduces the required alignment precision between the light beam and the detector, where the alignment tolerance is now limited only by the photodetector size.
Still further objects, features and advantages of the present invention will become apparent upon a reading of the following detailed description of a preferred form of the invention.